The relaxin knockout (rlx −/−) mouse was used to assess the effect, during pregnancy, of relaxin with regard to water, collagen content, growth, and morphology of the nipple (N), vagina (V), uterus, cervix (C), pubic symphysis (PS), and mammary gland (MG). The results presented here indicate that during pregnancy, relaxin increases the growth of the N, C, V, and PS. Large increases in water content in the PS (20%) occurred in pregnant (Day 18.5) wild-type (rlx / ) mice but not in rlx −/− animals. This indicates that in the PS, relaxin might increase the concentration of a water-retaining extracellular matrix component (hyaluronate). In the pregnant rlx / mouse, collagen content decreased significantly in the N and V but not in other tissues. There were no significant changes in the rlx −/− mouse. This contrasts with findings in the rat, in which relaxin has been found to cause decreases in collagen concentrations in the V, C, and PS. Histological analysis showed that the collagen stain was more condensed in the tissues (V, C, PS, N, and MG) of rlx −/− mice than in those of rlx / mice. This phenomenon indicates that the failure of collagen degradation and lack of growth in the N underlie the inability of the rlx −/− mice to feed their young, as reported previously. Vaginal and cervical luminal epithelia, which proliferated markedly in the rlx / pregnant mice, remained relatively atrophic in the rlx −/− mice. As proliferation and differentiation of uterine and vaginal epithelia are thought to be induced by a paracrine stromal factor that acts upon estrogen stimulation, our results indicate that relaxin may be this paracrine factor.